Federal prosecutors are targeting more than 100 marijuana shops in Los Angeles County this week, threatening prosecution if dispensary owners stay in operation. Officials also moved to seize two properties in Long Beach catering to marijuana users.

Letters from the federal government were sent to dispensaries in Los Angeles, Long Beach, Lancaster, and Pearblossom, U.S. Attorney’s Office spokesman Thom Mrozek said. The letters informed dispensary owners they are operating in violation of federal laws.

In Los Angeles, 71 dispensaries in South L.A., downtown, and the Harbor area were sent notices, Mrozek said.

The government’s actions represent the latest effort to enforce federal laws and the newest challenge to California’s 17-year-old, voter-approved law allowing the sale of marijuana as a medicinal treatment. Federal authorities contend the 1996 legislation approved by California voters was intended to allow small, nonprofit collectives for sick patients, and not result in an explosion of pot stores.

“Anyone who has spent any time in a marijuana store can tell these are drug-trafficking businesses,” Mrozek said. “All the stores we have seen are generating significant amount of profits.”

The letters sent Tuesday come weeks after Los Angeles voters approved Measure D, which allows 135 dispensaries — those facilities that were already open and registered with the city before a 2007 moratorium — to stay open. All other dispensaries must shut down.

While the Los Angeles City Attorney’s Office is finalizing the list of 135 dispensaries, and expects to release the list in about 10 days, Mrozek said he expects some overlap between facilities ordered to shut down and those on the city’s list allowed to stay open.

Deputy City Attorney William Carter said his office will be informing any dispensary on the city’s list they must comply with federal laws.

Such is the confusing state of marijuana laws in California, where federal and state rules often conflict with one another as local governments take a piecemeal approach to regulating pot.

Kris Hermes, spokesperson for the Americans for Safe Access, a medical marijuana advocacy group, criticized the federal government’s actions Tuesday, saying that he believes stores targeted by the U.S. Attorney’s Office should be allowed to operate because they are in compliance with state laws.

“Thousands of people will be left without safe and legal access to medical marijuana,” Hermes said.

In Long Beach, where dispensaries have been illegal since last year, the city’s police chief praised the crackdown, saying that shops have been a problem for the city.

“We always welcome the opportunity to partner with federal authorities in an effort to address these illegal operations that affect the quality of life in our community,” Long Beach Police Chief Jim McDonnell said in a statement.

The federal government has also filed two asset forfeiture lawsuits in Long Beach where officials said marijuana stores are currently operating.

The forfeiture lawsuits allege the owners of the properties allowed commercial marijuana stores or growing operations. The dispensaries named by the U.S. Attorney’s Office are the Healing Tree Holistic Association on East Anaheim Street and the Naples Wellness Center on East 2nd Street.

Messages left for owners at the two stores were not immediately returned.